my 97 4.0l inj wrangler has developed a fault ....

it starts but as soon as i press the throttle it stutters and
backfires.

plugs changed, ignition leads and distributer cap changed
no change

the car has been converted to dual fuel .....lpg/petrol

it was working fine on lpg.......but then developed this fault at
start up

now its on both.......

could it be a sensor in the carb??...if so which one?

can you clean these.or is it a replacment?

could it be the sensor was on it way out .....when on lpg it worked
(as lpg is thinner then petrol)

but now its knackered?